Portable electronic equipment such as mobile phones and personal digital assistants (PDA) usually use rechargeable batteries. Power adaptors (or AC-DC power converters) are traditionally used to charge the batteries in the electronic equipment. Due to the wide range of portable electronic products, many people nowadays have a wide range of power adaptors because there is no standard for charging different types of portable electronic equipment.
Recently, a number of types of planar inductive charging platforms have been proposed. One example is described in GB2399225A which generates an AC electromagnetic flux 1 with the flux lines flowing “horizontally” along the charging surface 2 as shown in FIG. 1A. A distributed winding is used in this charging platform for generating the AC flux. This principle is in fact similar to the AC electromagnetic flux generated in a cylindrical motor, except that the cylindrical structure is compressed into a flat pancake shape. As the flux needs to flow horizontally along the upper and lower surfaces, two inherent limitations arise.
Firstly, an electromagnetic flux guide must be used to guide the flux along the bottom surface. This is usually a layer of soft magnetic material such as ferrite or amorphous alloy. In order to provide sufficient flux, this layer must be “thick” enough so that the flux can flow along the layer of soft magnetic material without magnetic saturation. Secondly, a similar problem applies to the secondary device that has to pick up to flux (and energy) on the upper surface of the charging platform.
FIG. 1B shows the device required for the charging platform of FIG. 1A. This consists of a magnetic core 3 and a winding. In order for the winding to sense the AC flux, the flux must flow into the cross-sectional area (shaded in FIG. 1B). Therefore, this cross-sectional area must be large enough so that enough flux and energy can be picked up by the secondary device. It should be noted that this secondary device must be housed inside the electronic equipment to be charged on the charging platform. The thickness of the secondary device is crucial to the applicability and practicality of the device. If it is too thick, it simply cannot be housed in the electronic equipment.
Another type of planar inductive battery charging platform is described in GB2389720A. Unlike GB2399225A, the charging platform described in GB2389720A uses a multilayer planar winding array to generate an AC flux 4 that has almost uniform magnitude over the entire charging surface 5. The lines of flux of this charging platform flow “perpendicularly” in and out of the charging surface as shown in FIG. 2. This perpendicular flow of flux is very beneficial because it allows energy transfer over the surface on which the electronic equipment (to be charged) is placed.
For both planar charging platforms described above, it is necessary to use an electromagnetic shield 6 on the bottom surface. If the charging platform is placed on a metallic desk, the AC flux generated in the charging platform may induce currents in the metallic desk, resulting in incorrect energy transfer and even heating effects in the metallic desk. U.S. Pat. No. 6,501,364 has been shown to be an effective electromagnetic shield for this type of planar charging platform. The electromagnetic shield in U.S. Pat. No. 6,501,364 simply consists of a thin layer of soft magnetic material (such as ferrite) and a thin layer of conductive material (such as copper).
Regarding energy transfer from the planar surface, one coreless printed-circuit-board (PCB) transformer technology pioneered by Hui and Tang [EP935263A; Chung, H., “Coreless printed-circuit board transformers for signal and energy transfer,” Electronics Letters, Volume: 34 Issue: 11, 28 May 1998, Page(s): 1052-1054; Hui, S. Y. R.; Henry Shu-Hung Chung; Tang, S. C., “Coreless printed circuit board (PCB) transformers for power MOSFET/IGBT gate drive circuits,” IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 422-430; Tang, S. C.; Hui, S. Y. R.; Henry Shu-Hung Chung, “Coreless printed circuit board (PCB) transformers with multiple secondary windings for complementary gate drive circuits,” IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 431-437; Hui, S. Y. R.; Tang, S. C.; Henry Shu-Hung Chung, “Optimal operation of coreless PCB transformer-isolated gate drive circuits with wide switching frequency range,” IEEE Transactions on Power Electronics, Volume: 14 Issue: 3, May 1999, Page(s): 506-514; Tang, S. C.; Hui, S. Y. R.; Henry Shu-Hung Chung, “Coreless planar printed-circuit-board (PCB) transformers-a fundamental concept for signal and energy transfer,” IEEE Transactions on Power Electronics, Volume: 15 Issue: 5, September 2000, Page(s): 931-941] has been proven to be an effective way.
Based on two planar windings on two parallel planes as shown in FIG. 3, it has been shown that both energy and signal can be transferred from one planar winding to another. This planar PCB transformer technology has been applied in a range of applications. In 2004, it was used by for a contactless battery charger for mobile phone in Choi B., Nho J., Cha H. and Choi S., “Design and implementation of low-profile contactless battery charger using planar printed circuit board windings as energy transfer device,” IEEE Transactions on Industrial Electronics, vol. 51, No. 1, February 2004, pp. 140-147. Choi et al uses one planar winding as a primary charging pad and a separate planar winding as a secondary winding as shown in FIGS. 4A and 4B. FIG. 5 shows the equivalent electrical circuit diagram of this contactless charging system. As explained in Choi et al, the circuit operation of the coreless PCB transformer is based on the theory proposed by Hui et al. It should be noted that the primary circuit is based on the resonant circuit described in Hui et al, while the front power stage of the secondary circuit is a standard winding with a diode rectifier that provides the rectified DC voltage for the charging circuit.
Two main problems suffered by the charging system of FIG. 5 proposed by Choi et al include: (1) The planar winding of the secondary module must be placed directly on top of the planar winding of the primary unit. If it is slightly misplaced, the energy transfer will be seriously hampered; (2) The use of one spiral planar winding in the secondary module to pick up energy emitted from the primary winding requires the choice of switching frequency to be very high. In Choi et al, the operating frequency has to be 950 kHz. Such high switching frequency leads to high switching loss in the primary inverter circuit, high AC resistance in the PCB copper tracks and more importantly high electromagnetic interference (EMI) emission.
Problem (1) can be solved by using a planar inductive charging platform based on a multi-layer planar winding array structure, which allows the charged electronic equipment to be placed anywhere on the charging surface as described in GB2389720A. However requiring a multi-layer charging platform increases the complexity of the charging platform undesirably.
A planar inductive battery charging platform that generates magnetic field with lines of flux flowing perpendicular to the planar surface (FIG. 2) can be constructed in two ways. The first and simplest way is to excite a coil with an AC power source as shown in FIG. 6A. A second method is to use a multi-layer winding matrix structure as shown in FIG. 6B similar to that described in GB2389720A. However, in both cases, it has been pointed out (Liu, X.; Chan, P. W.; Hui, S. Y. R.; “Finite element simulation of a universal contactless battery charging platform,” IEEE Applied Power Electronics Conference 2005, APEC 2005, Volume 3, 6-10 Mar. 2005 Page(s): 1927-1932) that a central voltage sag phenomenon exists. In practice, a secondary module (or energy receiving element) is used to pick up the energy for charging the load. FIG. 7 shows that in one practical experimental setup, the rectified DC voltage picked up by a secondary module on the planar surface is not entirely uniform over the planar surface and in particular is reduced in the central part of the planar surface. This is known as central voltage sag phenomenon. This voltage sag increases as the surface area of the charging area increases.